Since a cold work tool is used in contact with a hard workpiece, the tool is required to have a sufficient hardness and wear resistance to resist the contact. Conventionally, alloy tool steels, such as SKD10 or SKD11 series pursuant to the JIS, have been used for a cold work tool material.
Typically, a cold work tool material is manufactured from a raw material, as a starting material in a form of an ingot or a bloom which is produced from the ingot. The starting material is subjected to various hot workings and heat treatments to produce a predetermined steel material, and then the steel material is subjected to an annealing process to produce a final material. Typically, the material in the annealed condition having a low hardness is supplied to a manufacturer of a cold work tool. The material supplied to the manufacturer is machined into a shape of the tool by cutting, boring or the like, and thereafter quenched and tempered to adjust it to have a predetermined hardness for use. After the adjustment of the hardness, finishing machining is typically conducted. Here, the term “quenching” refers to an operation for heating a cold work tool material, after machined in a shape of the tool, at an austenitic phase temperature range and then rapidly cooling it to transform a structure thereof into a martensitic structure. Thus, the material has such a composition that can have a martensitic structure by quenching.
In this connection, “dimensional change through heat treatment” may occur in the cold work tool material. The “dimensional change through heat treatment” means a volume (dimension) change between before and after the quenching and tempering. Particularly, the dimensional change in a direction extended by hot working (that is, in a longitudinal direction of the material) is an expanding change that occurs through the quenching, and the expansion is largest in the direction. If the large expansion occurs in the longitudinal direction of the material, dimensional control by tempering becomes difficult. Typically, the cold work tool material shrinks through a low temperature tempering, while it expands through a high temperature tempering. Thus, the tempering is conducted at a temperature where the dimensional change becomes nearly zero relative to the annealed material, when the dimensional change should be controlled for the cold work tool. However, the large expansion in the longitudinal direction (that is anisotropic to width and thickness directions) during quenching is hardly cancelled by the tempering step. Therefore, it is required to design a complicated “cutting allowance” for finish machining of the shape before the quenching and tempering. If the expansion in the longitudinal direction is too large, adjustment by the “cutting allowance” becomes impossible.
A cold work tool material including a reduced amount of large carbides have been proposed to the problem, on assumption that the dimensional change through heat treatment occurs due to the large carbides in a structure of the material. For example, JP-A-2001-294974 proposes a cold work tool material having a cross-sectional structure in which carbides having an area of 20 μm2 or larger occupy an area ratio of 3% or less after quenching and tempering (see Patent Literature 1). Also, JP-A-2009-132990 proposes a cold work tool material having a cross-sectional structure parallel to an direction extended by hot working, in which carbides having a circle equivalent diameter of 2 μm or greater have an area ration of 0.5% or less before quenching and tempering, for the purpose of suppressing the expansion in the longitudinal direction (see Patent Literature 2).